Consider a system operating with an integration time of 60s (link refresh period), \(QBER_I=0.5\%\) , \(P^A_X=P^B_X\)  and repetition rate of 100MHz. A plot showing the region where one obtains secret keys is shown in figure REF  where we have numerical optimised all protocol parameters at each point, as discussed in section REF . The shaded region shows where one obtains a nonzero value for the secret key. We observe that \(\ell \) , when non-zero, is large, even for relatively large losses. An important feature of the plot is that a slight increase in \(p_{ec}\)  or \(\eta _{\mathrm {loss}}\)  can cause a sharp drop off in the secret key or reduce it to zero. This is illustrated more clearly in figure REF  where we plot \(\ell \)  against \(\eta _{\mathrm {loss}}\) , for different values of \(QBER_I\)  and \(p_{ec}\) . An important feature is the sharp drop of \(\ell \)  to zero resulting from finite statistical corrections [1]}.
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